A storage apparatus (e.g. a disk array apparatus) generally comprises multiple HDDs (hard disk drives) and a controller which controls access to the multiple HDDs (hereinafter, a disk controller is abbreviated to a “DKC”). This type of storage apparatus usually comprises one or more RAID groups configured of multiple HDDs. The DKC, in accordance with the RAID level of the RAID groups, accesses the RAID groups.
As a RAID level, for example, RAID 1 exists (Patent Literature 1). A RAID group to which RAID 1 is applied is configured of a master HDD and a mirror HDD. The DKC receives write target data from a host apparatus, and duplicates the write target data. Then, the DKC writes one type of the duplicated data to one HDD (hereinafter referred to as a master HDD), and writes the other type of data to the other HDD (hereinafter referred to as a mirror HDD).